1. Field of the Invention
This invention relates to improvements in MSK modulation techniques, and more particularly to a method for generating an MSK detectable signal having reduced side-lobe energy.
2. Description of the Prior Art
Minimum shift keying (MSK) modulation has become of increasing interest and widespread use, since it provides a way to transmit data at relatively high rates in a given bandwidth and with maximum signal-to-noise performance. MSK modulation was first introduced and explained in U.S. Pat. No. 2,977,417 to M. L. Doelz, et al., assigned to the assignee hereof.
Briefly, an MSK modulated signal can be thought of as two combined orthogonal signals or channels plus or minus 90 degrees out of phase with each other, each of which is phase reversal keyed to represent alternate bits of a binary signal desired to be transmitted.
Each keyed pulse period, therefore, has a duration of two bit periods. These pulse periods are staggered in time by one bit period. After the two channels are phase shift keyed, they are each amplitude modulated with a one-half sinusoid, then combined by addition. Since the sine shaped envelopes of the two channels are 90 degrees out of phase with one another, the sum of the two channels results in a signal with constant envelope amplitude, which can be amplified, if desired, with, for example, a non-linear Class-C amplifier for transmission.
Various demodulation methods have been advanced for an MSK modulated signal. One method is to separate the incoming signal into two channels, which are each multiplied by a carrier having the same phase as each of the generator carriers. The plus and minus d-c signals are then multiplied by a sine shaped weighting signal, and the result applied to an integrator for the duration of the pulse period. The polarity of the integrator output determines whether the bit was a "1" or a "0". The demodulated bits from the two orthogonal channels are then interleaved to produce an output data bit stream. MSK modulated signals can also be detected by employing appropriately designed passive filters, as is known in the art.
One of the attributes of the above described MSK signal is that it has a particular frequency spectrum associated with it.
The average spectral density pattern of an MSK emission is shown in FIG. 1, representing a long term average, with random keying. (It is also the spectral density of a one-half sinusoid pulse and, additionally, this spectral density pattern represents the passband of an ideal matched-filter MSK detector.) The equation for this spectral density envelope pattern is: ##EQU1## Where A is the voltage amplitude of the MSK wave,
R is bits per second and, PA1 f is the frequency separation from the carrier frequency.
The spectral density curves can be normalized by letting the amplitude by unity when f=0 and scaling frequency as f/R.
An MSK wave is also identical to that of FSK with a modulation index of exactly 0.5. This means that the two FSK frequencies are separated by R/2 which places them plus and minus R/4 from the center, or carrier, frequency. The keying pattern is different, however, because with MSK the signal changes frequency when the next bit is the same as the previous bit. With FSK the upper frequency usually is designated to represent a "0" and the lower frequency a "1".
The MSK wave changes frequency instantly, but there is no break in phase continuity. Curve 20 of FIG. 2 illustrates MSK frequency changes for a given series of data bits, and is described below in detail.
The phase of the MSK wave (relative to the phase of the carrier) is illustrated by the dotted line 30 of FIG. 3 for the same bit pattern, also described below in detail. The phase changes 90 degrees during each period and the slope of the phase curve changes sign at the instant of a frequency change.
FIG. 4 shows a phasor diagram useful in analyzing an MSK wave. The phasor has a constant amplitude and rotates in phase, relative to carrier phase, by exactly plus or minus 90 degrees during each bit period. The i and q components of the phasor represent the amplitudes of each of the orthogonal channels.
Several investigators have described various means to bandlimit an MSK signal, usually with different objectives and self-imposed constraints. Early attempts to bandlimit very low frequency MSK emissions by employing bandlimiting filters in the low power exciter of a high power transmitter were only partially successful. Bandlimiting the MSK signal with conventional filters produces an amplitude ripple on the signal. Conventional Class C power amplifiers would remove the ripple by amplitude limiting the signal which partially restores the sidelobe emission. Band limiting filtering has been successfully employed at the output of low power microwave transmitters, however. For example, one invention is designed to achieve the maximum transmition bit rate within a given channel bandwidth as defined by the FCC. Filtering schemes as used in the prior art are not practical because of excessive size, weight, power loss and cost when used in the output of a high power transmitter. Furthermore, when filtering is done in the exciter or low power stages, the undesired emission is regenerated to an undesirable extent.
Another investigator adds certain levels of odd harmonic components to the sine-pulse shape of the orthogonal channels. This maintains a constant amplitude signal which can be amplified with a nonlinear RF amplifier without regenerating the sidelobes. This technique causes the sidelobes beyond plus and minus three bit rates from the carrier to fall off much faster, but it does not help at the plus and minus three bit rate frequency.
Often there is a need to receive a signal in the presence of a very strong signal on a nearby frequency channel. One particular need at VLF frequencies is the ability to receive a relatively narrow band signal located just beyond three times the data bit rate of the strong signal from the strong signal carrier frequency. The spectral density of an unmodified MSK signal is down to about -40 dB at that location, but it would be very advantageous to reduce it to a level of -80 dB or lower.